Device for stabilizing the gain of photomultiplier tubes



19671 A. M. GOUYON ETAL 2,994,782

DEVICE FOR STABILIZING THE GAIN OF PHOTOMULTIPLIER mass Filed Dec. 14,1959 F5 Ty. 2

2 Sheets-Sheet 1 Vn- 7 V0 V0 +7 Vd V I I l 800 1000 1200 1400 V ug- 1,1961 A. M. GOUYON ETAI. 2,994,782

DEVICE FOR STABILIZING THE GAIN 0F PHOTOMULTIPLIER TUBES Filed Dec. 14,1959 2 Sheets-Sheet 2 United States Patent 2,994,782 DEVICE FORSTABILIZING THE GAIN 0F PHOTOMULTIPLIER TUBES Arthur Marcel Gouyon,Saint-Egreve, Marcel 'Kermagoret, Paris, and Andre Pascal,Boulogne-sur-Seine, France, assignors to Commissariat a IEnergieAtomique, Paris, France, an organization of France Filed Dec. 14, 1959,Ser. No. 859,531 Claims priority, application France Dec. 16, 1958 5Claims. (Cl. 250-207) The present invention relates to photomultipliertubes, that is to say to tubes for transforming incident photons intoelectrical pulses adapted to be used for measuring the number and/or theenergy of these photons.

It is known that a photomultiplier tube comprises, inside an evacuatedvessel: a photocathode emitting a bunch of electrons in response to anincident photon striking said photocathode, a series of intermediateelectrodes or dynodes every dynode emitting, in response to the primaryelectrons coming from the preceding electrode (photocathode or dynode)that strike said first mentioned dynode, a great number of secondaryelectrons (multi plication effect by secondary emission), and acollecting electrode or anode which receives the electrons emitted bythe last dynode and supplies the electrical pulses to be used.

In order to permit emission and collection of the electrons and toestablish the electrostatic field which ensures ,the displacement of theelectrons, use is generally made of a high voltage feed source insertedbetween the anode and the photocathode and of a voltage divider devicealso inserted between the anode and the photocathode, the respectivedynodes being all fed from said voltage divider so as to be brought topotentials which increase progressively in the order in which they aredisposed between the photocathode and the anode.

In view of the fact that the total amplification or gain of aphotomultiplier :tube having n amplification stages is substantiallyequal to the nth power of the amplification per stage and that this lastmentioned amplification depends directly upon the potential differencebetween two successive dynodes, which difference is substantiallyproportional to the total potential difference, the gain of aphotomultiplier tube varies very quickly as a function of the supply orfeed high voltage. It is known for instance that variations of 1% of thefeed high voltage produce gain variations ranging approximately from 8to 12% and that it is difficult, in industrial applications of thisapparatus, to use high voltage feed sources having a stabilitycorresponding to maximum variations lower than 1%.

The object of the present invention is to achieve a stabilization of thegain of photomultiplier tubes making use of feed sources, even littlestabilized (having for instance voltage variations which may be as highas through means which are simple, efiicient, unexpensive and requireonly an easy adjustment, which is quite necessary for industrialapplications of photomultiplier tubes (measurement of X-rays intensity,counting of nuclear particles in combination with scintillators, and soon).

Therefore, the present invention for stabilizing the gain of aphotomultiplier tube including n dynodes, n being at least equal to two,located between a photocathode and an anode, said tube being fed from ahigh voltage source having its two terminals connected respectively tosaid photocathode and said anode and also from a voltage dividerconnected between said terminals of said source, consists in feedingonly n-l dynodes in the known manner from said voltage divider andfeeding the remaining dynode with a stabilizing voltage derived fromsaid high voltage and which is an increasing linear function of saidhigh voltage.

A device according to the present invention for stabilizing the gain ofa photomultiplier tube and embodying the above stated method, comprises,for deducing from said high voltage a stabilizing voltage to be fed tosaid remaining dynode and having a value which is an in= creasing linearfunction of said high voltage, a network disposed between said terminalsof said high voltage source, said network comprising in series at leastone first resistor, a tube capable of maintaining a constant differ.-ence potential between its terminals and at least one second resistor,at least one of said resistors being adjustable, and a potentiometerconnected in shunt with said last mentioned tube, said potentiometerincluding a slider connected with said remaining dynode to supply itwith its feed voltage.

The adjustment of this device is very simple because the adjustableresistor ensures once and for all the automatic regulation of the gainof the photomultiplier, the potentiometer being only used for adjustingthe value of said gain Without disturbing the other characteristics.

In the preferred embodiment of this invention the tube capable ofmaintaining a constant potential dilference between its terminals is avoltage stabilizing tube of the corona eifect type and the dynode fedfrom the potentiometer is the fourth, fifth or sixth dynode from thephotocathode.

Preferred embodiments of the present invention will be hereinafterdescribed with reference to the accompanying drawings, given merely byway of example and in which:

FIGS. 1 to 4 show four curves representing the variations of the gain ofa photomultiplier tube as a function of the feed voltage and of thevoltage applied to a given dynode, these curves facilitating theexplanation of the principle of stabilization of the gain according tothe invention.

FIG. 5 is a diagram illustrating the gain stabilization of aphotomultiplier tube by the method according to the invention.

FIG. 6 shows a photomultiplier tube cooperating with a device madeaccording to the present invention for regulating the gain.

FIG. 7 shows curves of adjustment of the device of FIG. 6.

Before giving a detailed description of the method and device accordingto the invention, it is believed advisable to explain, with reference tothe curves of FIGS. 1 to 4, how the gain of a photomultiplier tubevaries and the chief causes of this variation when the feed high voltageof this tube varies.

When the feed voltage of one of the dynodes of a photomultiplier tube ismodified, there is produced a gain variation shown by FIG. 1. On thisfigure, the gain G is plotted in ordinates (with a logarithmic scale)and the voltage Vd of the dynode in abscissae. This figure shows thatthe same gain G may be obtained for two values (such as Vn-l and Vn+l)of the voltage Vd which are substantially symmetrical with respect tothe normal potential V of the nth dynode.

For this reason, if we represent, as on FIG. 2, a curve of constantgain, as a function on the one hand of the total feed high voltage V ofthe photomultiplier tube and on the other hand of the potential V of thenth dynode, it is found that such a curve comprises two branches whichare substantially rectilinear and therefore comply with an equation ofthe following type in which parameters a and b are constant factors.

The shape of such curves is essentially determined by the change of thesecondary emission coeflicient, the effect of which can be calculated.As a matter of fact, if the potential of the nth dynode is modified,there is obtained k being a constant.

FIG. 3 shows, with the same references as on FIG. 1, the variations ofthe gain of a photomultiplier tube due to the modification of potentialof the nth dynode, resulting exclusively from the variations of thesecondary emission coefi'icient; the curve in dotted lines shown on thesame figure represents the gain variation that has been experimentallyobserved.

The difference between these two curves is small and corresponds to adefocusing of the electron beam by modification of the interdynodepotential which produces a loss of the electrons.

It is therefore found that actually the effects of the variation of thesecondary emission are the most material.

If we show on the same figure (FIG. 4) a number of constant gain curves(the value of the gain being indicated on every curve), still as a.function of this voltage V,, applied to the dynode and of the totalvoltage V (these voltages being expressed in volts), it is seen that,when the gain changes, the slopes of the lower and upper branches of thecurves are maintained. It follows that the ordinate, at the origin, ofthese straight lines varies by an amount DV proportional to the increaseDV of the total voltage. Consequently, if a regulating network servingto feed the nth dynode is given a slope such a network would make itpossible to obtain a stabilization of the gain of the photomultipliertube despite variations of the feed voltage for different values. Themethod and device according to the invention are based upon this basicfact.

We will first refer to the diagram of FIG. 5 which shows aphotomultiplier tube comprising, in an evacuated vessel 1 provided witha transparent window 1a, a photocathode 2, a series of dynodes 3 and acollecting electrode or anode 4. The electrodes 2, 3 and 4 of this tubeare fed from a high voltage source 5, the postitive terminal of which isconnected with anode 4, the negative terminal, connected to photocathode2, being grounded and from a voltage divider 6 (constituted by a numberof resistances in series) inserted between photocathode 2 and anode 4,this voltage divider serving to the feed of dynode 3 with the exceptionof a single one, to wit dynode 3a.

According to the main feature of the present invention, this dynode 3ais fed with a voltage which is an increasing linear function of thevoltage delivered by source 5. For this purpose, we insert betwen theterminals of source 5, across the voltage divider, a network includingin series a first resistor 7, a tube 8 maintaining a constant potentialdifference between its terminals and a second resistor 9. The feedvoltage of dynode "3a is collected through a potentiometer 10 shuntingtube 8, the two terminals of this potentiometer being connected topoints 11 and 12, whereas its slider 13 is connected to dynode 3a.

The characteristics of the arrangement of FIG. 5 do comply with EquationI which represents the upper and lower branches of the constant gaincurves.

This is due to the fact that, within the operative range of tube 8, thevoltage across the terminals of this tube remaining constant and V beingthe high voltage of source 5,

V the voltage across the terminals of tube 8 between 11 and 12, a

R the resistance of resistor 7,

R the resistance of resistor 9,

V the potential of dynode 3a,

i the intensity of the current through resistors 7 and 9, V the groundpotential,

V the potential of slider 13 of potentiometer 10,

1+ 2) o and if:

V =V +m-V (with m ranging from 0 to l) we have:

V V V =R i+mV As 2 c v V,

we obtain:

R f RIZ RP 121+ R2) By identifying Equations I and IV, we obtain:

a= and b=V,,(m-a) which shows that the slope of the regulation straightline is maintained when m, that is to say the gain of the tube, ischanged.

However, it is necessary to make a suitable choice of:

(a) tube 8 so that it permits a relatively large modification of voltagefor the regulated dynode, without exceeding 1,000 volts; preferably acorona efiect tube is used; we might also make use of a neon tube butthere are two important drawbacks inherent in this kind of tubesz-itslow regulation voltage volts) and its high consumption.

(11) the regulating dynode 3a; the current of this dynode must not betoo high which would exclude the possibility of using the last dynodes;the fourth, fifth or sixth dynode is quite suitable.

Account being taken of the above remarks, in the embodiment illustratedby FIG. 6 (which concerns a photo multiplier tube 1 having ten dynodes,nine of which, designated by reference numeral 3, are fed from a set 6of resistors 6a in series, whereas the remaining dynode 3a, which is thefourth dynode is fed from a network according to the invention), thevoltage stabilization tube 8 is constituted by a corona effect tube andresistor 7 is divided into two portions, to wit a fixed resistor 7a andan adjustable resistor 7b (consisting for instance of a rheostat); in alikewise manner, potentiometer 10 is advantageously replaced by a seriesarrangement including, in addition to a potentiometer 10b, two fixedresistors 10a and 100, which makes it possible'to use a potentiometer 1%adapted for a narrower resistance band.

Consequently, in the arrangement of FIG. 6, R will include resistors 7aand 7b and m is no longer variable between 0 and l, but between twovalues m m such as m 0 and m' 1.

Calculation of the values of the resistances to be used in the gainstabilizing arrangement according to the invention may be effected inthe following manner.

Starting from the constant gain curves (FIG. 4) of the photomultipliertube, we choose a point of operation as a function of V and V,,; thispoint of operation must be necessarily chosen on a rectilinear portionof the constant gain curve such that the feed voltage V remains, despitepossible variations, on the rectilinear portion of this curve. Forinstance, if a point of operation is chosen for a voltage V=1,700 voltswith possible variation of the constant gain curve must be rectilinearbetween 1,530 and 1,870 volts.

Knowing V and V (voltages across the terminals of corona tube 8), R +Ris deduced therefrom. Then, starting from the main current i and fromthe mean cur- Resistance of resistor a=17 M ohm. Resistance of resistor10c=2 M ohm. Resistance of potentiometer 10b=12 M ohm.

The adjustment of the gain may be performed very quickly in thefollowing manner:

We trace a curve (FIG. 7) which represents the amplitude A in volts(which can be read 'for instance on a voltmeter located at 14, FIG. 6,at the extremities of a not shown resistor) obtained in response to aconstant luminous phenomenon, as a function of the feed voltage V applied to the photomultiplier tube. If the correction is too great (curveC the slope of the regulating system is too small and the value of Rmust be increased by acting upon rheostat 7b (FIG. 6); on the contrary,if the correction is not suflicient (curve C3), the value of R isreduced so as to obtain finally a curve comprising a horizontal portion(curve 0,).

The adjustment being finished, it suflices to actuate slider 13 ofpotentiometer 10b ('FIG. 6) to obtain the desired gain.

We thus obtain excellent results since for a variation of the feedvoltage of -10% the relative variation of gain is about 1%.

We thus achieve in very good conditions the desired stabilization of thegain of a photomultiplier tube, which makes it possible to use suchtubes without either stabilized feed sources, which are very expensiveand of very great dimension, or to known gain stabilizing arrangementsrequiring long and delicate adjustments (which cannot be made inindustrial utilisations) due to the dispersion of the characteristics ofthe photomultiplier tubes and of the regulating elements associatedtherewith and due to the fact that adjustment of the stabilizationreacts upon the gain and conversely, which requires an adjustment bysuccessive approximations.

In a general manner, while we have, in the above description, disclosedwhat we deem to be practical and eflicient embodiments of our invention,it should be well understood that we do not wish to be limited theretoas there might be changes made in the arrangement, disposition and formof the parts without departing from the principle of the presentinvention as comprehended within the scope of the accompanying claims.

What we claim is:

1. For use with a photomultiplier tube including a photocathode, ndynodes located behind one another after said photocathode and an anodelocated behind said dynodes, a high voltage source having its twoterminals connected respectively to said photocathode and said anode anda voltage divider device connected between said source terminals, adevice for stabilizing the gain of said photomultiplier tube whichcomprises conventional means for feeding n-l of said dynodes from saidvoltage divider and means for supplying the remaining dynode with astabilizing voltage derived :from said high voltage and the value ofwhich is an increasing linear function of said high voltage, said lastmentioned means including a network disposed between said terminals ofsaid high voltage source, said network comprising in series at least onefirst resistor, a tube capable of maintaining a constant potentialdifference between its terminals and at least one second resistor, atleast one of said resistors being adjustable, and a potentiometerconnected in shunt with said last mentioned tube, said potentiometerincluding a slider connected with said remaining dynode to supply itwith its feed voltage.

2. A device according to claim 1 in which said tube maintaining aconstant potential difference between its terminals is a corona efiecttube.

3. A device according to claim 1 in which n is an integer close to ten,said remaining dynode being the fourth starting from said cathode.

4. A device according to claim 1 in which n is an integer close to ten,said remaining dynode being the starting from said cathode.

5. A device according to claim 1 in which n is an integer close to ten,said remaining dynode being the sixth starting from said cathode.

No references cited.

